1. Field of the Invention
The present invention relates to a semiconductor laser.
2. Description of the Prior Art
A semiconductor laser has distinct advantages such as smallness in bulk, high efficiency and direct modulation by means of its current, and therefore has a bright future as a light source for optical communication, optical data processing or the like. Lasers, of such use, necessitate having the characteristic of stable fundamental transverse mode lasing; and especially fluctuation of light intensity (noise) is an important problem when the laser is used for transmission of information by for instance modulating the laser light.
The conventional continuous wave (CW) lasing semiconductor laser has double heterostructure (DH structure), and cleaved faces are used as the end faces of its cavity. On the other hand, as a waveguide to provide guiding in the junction plane, a gain guide or refractive index guide has been used.
FIG. 1 is a sectional front view showing an exemplary configuration of the conventional planar stripe type semiconductor laser of the gain guide type, and FIG. 2 is a sectional front view showing an exemplary configuration of the conventional channelled substrate type semiconductor laser of the refractive index guide type.
In the above two conventional examples, the parts designated by the numerals 1 to 8 are as follows:
______________________________________ 1 Substrate n-GaAs, 2 a first clad layer n-Ga.sub.1-y Al.sub.y As, 3 an active layer undoped Ga.sub.1-x Al.sub.x As, 4 a second clad layer p-Ga.sub.1-y Al.sub.y As, 5 an isolation layer n-GaAs, 6 a positive side electrode, 7 a negative side electrode, and 8 a stripe shaped current Zn-diffused region injection region. ______________________________________
It is known that when the above-mentioned conventional semiconductor laser is operated to oscillate under a predetermined constant light output power, noise characteristics peculiar to the configurations are observed.
FIG. 3 is graph showing such noise characteristics, wherein the abscissa is graduated with ambient temperature and the ordinate is graduated with S/N ratios of 3 mW output and noise which is measured with the center frequency of 3 MHz and band width of 300 KHz. The curve I shows the characteristic of the planar stripe laser of FIG. 1 and the curve II shows that of the channelled substrate type laser of FIG. 2. As shown by FIG. 3, the gain guide type lasers such as the planar stripe type lasers have generally higher noise levels than the channelled substrate type lasers. This is because in the gain guide type lasers, there is no built-in distribution of refractive index to stabilize the transverse mode, and the distributions of the electromagnetic fields and of carriers are not temporally and spatially stable. Therefore, the noise becomes high as a result of the interaction of above-mentioned two factors. However, in the gain guide type lasers the longitudinal mode becomes a multiple longitudinal mode, and therefore, noise formed by competition of respective longitudinal modes is small. Accordingly, even when the gain spectrum change takes place as a result of temperature change, the noise level flicker due to the mode competition is small. On the other hand, in the refractive index guide type lasers, such as channelled substrate lasers, the noise level is lower in the stabilized noise range in comparison with that of the gain guide type laser, since the transverse mode thereof is stabilized. But, in general in the transverse-mode stabilized laser, the oscillation mode is of a single longitudinal mode. Accordingly, when the gain spectrum distribution changes responding to temperature change, the longitudinal mode jumps and the neighboring two longitudinal modes create the mode-competition between each other, and produce a high noise. Therefore, the refractive index guide laser generally has ranges of high noise which appear as a result of temperature changes. Therefore, if the conventional semiconductor lasers are used for a light source for playing back a video disk, due to their high noise characteristics, i.e. low S/N ratio, their uses are not practical.